The present invention generally relates to a continuously variable hydrostatic transmission, and more specifically to a method and apparatus for smoothing the output of a hydrostatic transmission near zero speed.
Many work machines, particularly earth working machines, use a continuously variable transmission to drive traction wheels or tracks which propel the work machine. Typically, a hydrostatic transmission, consisting of a variable speed hydraulic pump and a hydraulic motor, provides a continuously variable speed output to the wheels or tracks of the work machine. In particular, the speed output can be continuously varied by controlling the displacement of the hydraulic pump.
Typically, the pump is set to operate in positive displacements with a first actuator actuating on a control spool, and set to operate in negative displacements with a second actuator actuating on the control spool. When it is necessary to move between positive and negative displacements, or visa versa, the pump must pass through a point of zero displacement where the control is switched from the first actuator to the second actuator. If the second actuator is engaged too little and too slowly, then the transition will be slow and perceived as not supplying a smooth continuous variation in output speed. On the other hand, if the second actuator is engaged too much and too rapidly, then there can be a discontinuity or jump in the output speed of the hydrostatic system which is also perceived as poor performance in a continuously variable transmission.
In addition, in order to operate over a wide range of operating conditions, the engine torque is split between the hydrostatic transmission and a mechanical transmission. The mechanical transmission has a number of transmission ranges corresponding to different operating speeds of the work machine. The combination of the hydrostatic transmission and the mechanical transmission allows a continuously variable transmission to operate over a wider range of speeds than is possible using the hydrostatic transmission alone.
A drawback to using a continuously variable transmission with a number of transmission ranges is that the hydraulic pump typically transitions through zero displacement in each of the operating ranges. At higher machine travel speed speeds, the poor transitions between the positive and negative displacements have a greater impact on the output speed of the transmission. Thus, any slowness or discontinuity in power transfer through the hydrostatic system has a greater impact on the durability and perceived performance of the continuously variable transmission.
What is needed therefore is a method and apparatus for smoothing the output of a hydrostatic transmission near zero speed which overcomes the above-mentioned drawbacks.
In accordance with a first embodiment of the present invention, there is provided a method of controlling a transmission having a variable displacement hydraulic pump, a hydraulic motor operatively connected to the pump, and a speed sensor operable to sense the speed of an output shaft of the motor, a first actuator for setting displacement of the pump in a positive displacement range in response to a first actuator command, a second actuator for setting displacement of the pump in a negative displacement range in response to a second actuator command, and a controller operable to transmit first actuator commands to the first actuator and transmit second actuator commands to the second actuator in response to operator inputs. The method includes the step of receiving operator inputs which require the displacement of the pump to move from the positive displacement range to the negative displacement range. The method further includes the step of generating first actuator commands which cause the first actuator to set the displacement to zero. The method yet further includes the step of generating second actuator commands in response to the motor speed reaching a predetermined motor speed which cause the second actuator to set the displacement of the pump in the negative displacement region.
In accordance with a second embodiment of the present invention, there is provided method of controlling a transmission. The transmission has a variable displacement hydraulic pump, a first actuator for setting displacement of the pump in a positive displacement range in response to a first actuator command, a second actuator for setting displacement of the pump in a negative displacement range in response to a second actuator command, and a controller operable to transmit first actuator commands to the first actuator and transmit second actuator commands to the second actuator in response to operator inputs The method includes the step of receiving operator inputs which require the displacement of the pump to move from the positive displacement range to the negative displacement range. The method further includes the step of generating first actuator commands which cause the first actuator to set the displacement to zero. The method yet further includes the step of generating second actuator commands in response to the displacement reaching a predetermined displacement which cause the second actuator to set the displacement of the pump in the negative displacement region.
In accordance with a third embodiment of the present invention, there is provided a transmission assembly having a variable displacement hydraulic pump, a hydraulic motor operatively connected to the pump, a speed sensor operable to sense the speed of an output shaft of the motor, a first actuator for setting displacement of the pump in a positive displacement range in response to a first actuator command, a second actuator for setting displacement of the pump in a negative displacement range in response to a second actuator command, and a controller operable to transmit first actuator commands to the first actuator and transmit second actuator commands to the second actuator in response to operator inputs. Operator inputs require that the displacement of the pump to move from the positive displacement range to the negative displacement range. The controller generates first actuator commands which cause the first actuator to set the displacement to zero. The controller generates second actuator commands in response to the motor speed reaching a predetermined motor speed which cause the second actuator to set the displacement of the pump in the negative displacement region.